Radiation curable compositions which are curable to function as pressure-sensitive adhesives are known in the prior art. For example, liquid oligomer compositions (providing solventless coatings) containing unsaturated end groups can be coated on a substrate and cured rapidly by radiation such as electron beam or ultraviolet light radiation to form adhesive layers. In U.S. Pat. No. 4,022,926 to Keough et al. issued on May 10, 1977 a method is disclosed for making adhesive labels in which the adhesive and base layers are formed by radiation curing. The curable compositions for both layers according to that patent preferably comprise a polyurethane capped with residues of a hydroxyalkyl acrylate or methacrylate ester. The curable composition for the adhesive layer therein is prepared preferably by adding tackifying resins or agents to the curable compositions used for the base layer in order to obtain the needed adhesive properties.
In U.S. Pat. No. 5,087,686 to Ansell et al. issued on Feb. 11, 1992, radiation curable compositions for providing pressure-sensitive adhesives are described which comprise a polyurethane capped with residues of hydroxyalkyl acrylate or methacrylate which do not require the addition of tackifying resins or agents to obtain the needed adhesive properties in the cured composition. The improvements therein are based on formulating compositions to contain mono-alcohol residues which are saturated or contain unsaturated groups which do not polymerize with the acrylate or methacrylate residues when subjected to radiation. The non-polymerizable residues as used in that patent are residues which do not polymerize with isocyanate or unsaturated groups.
Known radiation curable pressure-sensitive adhesives available commercially will ordinarily favor tack, an important property in pressure-sensitive adhesives, at the expense of high temperature holding power. The invention described in the '686 patent to Ansell et al. lacks a commercially desirable balance of tack and high temperature holding power. Improved tack in typical pressure-sensitive adhesive formulations is obtained at the expense of poorer high temperature holding power. The formulations of the present invention provide adhesives with good tack and good or excellent holding power at high temperatures.